Cables with stranded or solid conductors coated with polymeric insulation have been used for many years. These cables are used for power transmission or distribution, telecommunications, and video and data transmission. The conductors of these cables may be made of aluminum or copper, with aluminum being prevalent in most cases due to lighter weigh and lower cost.
The cable construction may also include jacketing, metal sheaths, or shields and metal armors over the polymeric insulation to seal out moisture and prevent damage to the insulation. Common 600 V electrical distribution cables may include a stronger, tougher outer insulation layer, but usually do not include a metal sheath or armor. All these cables may be subjected to damage during or after installation, especially when directly buried, installed in tunnels, or inside buried pipes, from cutting by sharp tools, accidental dig in with shovels and picks, backfill with sharp rocks, and the like. The damage can be partial or total rupture of the outer sheath and possibly also of the insulating layer, with consequent infiltration of moisture and generation of leakage current. Further, if the rupture of the coating layers reaches the conductor, the combined effect of leakage current and moisture lead to a gradual corrosion of the conductor until complete breakage of the conductor occurs.
Aluminum conductors are most susceptible to rapid corrosion due to leakage currents. As oxides build up on the conductor, the diameter increases and the damage in the insulation is widened or opened, causing more water ingress. Eventually, the wires in the conductor corrode until few are left to carry a current, resulting in a cable failure.
To obtain effective protection against mechanical abuses, the cable can be provided with an outer structure capable of withstanding both cutting and compression. This outer structure comprises a sheath made of a metal or of a plastic material combined with metal armoring. Besides being expensive, this solution leads to a considerable increase in cable dimension and rigidity, thus making this solution unsuitable for cable which require easiness of installation and low cost, such as low-voltage cables.
Patent application DE 1,590,985 discloses a telecommunication or high-voltage cable which is protected from mechanical damage by means of an outer sheath having on its interior, micro-capsules containing a liquid which is capable of solidifying rapidly once the micro-capsule is broken. The solidifying material from the broken micro-capsule closes the accidental cut. A disadvantage is the high cost of introducing a large amount of micro-capsules into the sheath extrusion process. Further, during the various stages of a cable's life (manufacturing, storage, installation, use) the coating layers are inevitably subjected to compression, bending, and thermal cycles, which can lead to rupture of the micro-capsules. Therefore, when the cable is actually damaged, the micro-capsules are not available to effect the self-healing.
U.S. Pat. No. 6,184,473 discloses a cable with a self-sealing agent that is a flowable, low molecular weight polymer that is pumpable and flowable at 25° C. inside the stand and over the surface of the conductor. The disadvantage is that the material could flow out of the conductor when it is heated by high current during peak demand or by an electrical short circuit. The conductor also may also move toward one side of the insulation wall and force all the sealant to the opposite side, eliminating the self-healing protection in the area toward which the conductor moved.
EP 0 940 819 A1 discloses a cable with a self repairing agent that has controlled flowability that is under pressure due to the extrusion of the insulation over it. The disadvantage is that if there are several damages in a small area, the pressure will dissipates and all of the damage areas may not be repaired. Further, the flowable material also has the same disadvantages as that disclosed in U.S. Pat. No. 6,184,473 discussed above.
EP 1 081 720 A1 discloses a cable with a self-healing agent as in EP 0 940 819 A1 that has controlled flowability under pressure due to the extrusion of insulation over it. The self-healing agent is contained in channels in the insulation, and the insulation has anchoring portions on the conductor. The disadvantage of this solution is that there is less sealant in the anchoring portions and thus damages at the anchoring portions may not be properly sealed. The sealant may also flow inside the channels due to external pressure leaving voids in some areas next to the insulation, which could cause an electrical discharge with out an optional coating over the insulation. The material can also flow out of the conductor as in U.S. Pat. No. 6,184,473 discussed above.
WO 01/46965 A1; U.S. Pat. No. 4,703,132; and U.S. Pat. No. 5,010,209, each discloses a cable comprising water-swellable material in the coating for resisting the ingress of water to the conductor. The water-swellable material is applied either as a powder or as a mixture of a polymeric compound and a water-swellable powder. The major disadvantage of this system is the high cost of incorporating a water-swellable power into the cable coating.
Despite recent advances, there remains a need for a low cost cable that is capable of self-healing. Damages in the cable should be sealed before extensive corrosion occurs in the conductor. Furthermore, the cable should be constructed at a low cost, with a minimum of additional processing equipment. The instant invention provides such a cable and more.